1. Field of the Invention
The invention relates to a lifetime measurement of a semiconductor device, and more particularly, to a lifetime measurement of an ultra-thin dielectric layer.
2. Description of the Related Art
When the requirements for the integration of the integrated circuit increase, the requirements for an ultra-thin dielectric layer with low leakage current also increases. Generally, a dielectric layer having a thickness less than 50 .ANG. is called an ultra-thin dielectric layer. At such a thickness, the test of the reliability of the ultra-thin dielectric layer plays an important role in quality control.
Generally, the reliability of a dielectric layer can be obtained by performing a time-dependent dielectric breakdown (TDDB) test under a constant voltage stress (CVS). The applied stress voltage is far greater than the device's operating voltage. As shown in FIG. 1, in the TDDB test, the current increases with the stress time, and after a period, the current dramatically increases. The phenomenon is called hard breakdown (HBD).
In fact, in the TDDB test for an ultra-thin dielectric layer, the soft breakdown (SBD) and the stress-induced leakage current (SILC) are also observed as shown in FIG. 2. Furthermore, in the traditional TDDB test, the quality of the SILC curve is generally poor, and the SBD curve is not remarkable. However, if the SBD and the SILC are not taken into account, the reliability of an ultra-thin dielectric layer is easily overestimated. As a result, the ultra-thin dielectric quality cannot be efficiently estimated.
In Microelectronic and Reliability (1998, pp37-72), A. Martin et al. have mentioned that the SBD and the SILC of the ultra-thin dielectric layer may be more easily monitored at a lower stress voltage. However, they did not point out a preferred stress voltage for monitoring the SBD and the SILC.